Auditory cortical forward masking effects in squirrel monkeys with unilateral noise-induced hearing loss.

Forward masking provides a sensitive measure of temporal and spectral processing in the auditory system and reflects underlying inhibitory mechanisms in auditory cortex. The present study examined how unilateral, noise-induced hearing loss (NIHL) alters cortical forward-masking properties in the primary auditory cortex (A1) of squirrel monkeys. Multi-unit responses were recorded from A1 of two control and two NIHL animals six months after right-ear acoustic overstimulation (1 kHz, 136 dB SPL, 3 h). Sequential masker-probe tones were presented with a 10 ms interval while masker frequency was varied around each site's best frequency (BF). In control cortex, probe responses were predominantly suppressed by the preceding masker with strong forward suppression centered near BF. In contrast, NIHL animals exhibited markedly fewer suppressive sites and more facilitatory responses. Suppression strength and inhibitory bandwidth were reduced particularly for low-BF neurons near the 1 kHz lesion frequency. The shift from suppression-dominated to facilitation-dominated forward masking was correlated with the magnitude of asymmetrical hearing loss as assessed by the ABR threshold difference between the two ears. These results demonstrate that chronic NIHL reshapes cortical forward-masking dynamics by weakening suppression and enhancing facilitation, likely reflecting a loss of inhibitory strength and altered interneuron balance. The findings link psychophysical deficits in temporal and spectral resolution to identifiable cortical network changes following hearing loss.